One of the most surprising games I can remember
When I did pick this game up, I expected to be just a normal Mario Tennis game, where you can play with all the characters from the Mario universe in tournaments of various difficulties and lenghts. And boy was I surprised by what I encountered here.
This game makes you start with a complete new character, a normal human being, not related to the Mario universe. Putting you into a full story-focused-RPG-tennis game. That's just amazing, what a great idea and execution. The story is well written, with a decent sense of progress. You earn experience and can choose what to upgrade and all of that RPG gameplay.
Another thing that surprised me alot was the character and abilities variety, each character can have Power Shots, that are abilties that can be use both for a powerful shot or a "save a lost ball" type of deal. You keep going through rankings and improving your character, you can play singles or duos, depending on what you like better.
You can still play exhibition games without the story mode, or with a another GBA with a link cable. Also you can do some challenge-like training on the story mode, and then you unlock them to play around outside of it, with other characters. You keep unlocking new characters by each match you win throughout the story, what's awesome. I loved this game!